Cisatracurium besylate has the chemical name (1R,1′R,2R,2′R)-2,2′-[1,5-pentanediylbis[oxy(3-oxo-3,1-propanediyl)]]bis[1-[(3,4-dimethoxyphenyl)methyl]-1,2,3,4-tetrahydro-6,7-dimethoxy-2-methyl-isoquinolinium dibenzenesulfonate and is represented by the structural formula (I) below:

Cisatracurium besylate is the dibenzenesulfonate salt of 1R-cis,1′R-cis isomer of atracurium (i.e., two molecules of benzenesulfonate per one diammonium molecule of cisatracurium). Cisatracurium besylate is one of the 10 possible isomers of atracurium besylate and constitutes approximately 15% of that mixture (atracurium has four chiral centers, two on each half of the molecule, one at each of the nitrogen atoms and the other at position 1 in each of the tetrahydroisoquinolinium rings so theoretically there are expected 16 possible isomers; however, due to the symmetry of the molecule, the number of atracurium isomers is reduced to 10).
Cisatracurium besylate is a nondepolarizing neuromuscular blocking agent indicated for inpatients and outpatients as an adjunct to general anesthesia, to facilitate tracheal intubation, and to provide skeletal muscle relaxation during surgery or mechanical ventilation in the Intensive Care Unit (ICU). Cisatracurium besylate possesses an activity that is superior to atracurium besylate, with significantly less side effects.
Cisatracurium besylate is marketed in the United States and Europe by Glaxo Wellcome and Abbott Laboratories under the trade name Nimbex®, which is a sterile, non-pyrogenic aqueous solution that is adjusted to pH 3.25 to 3.65 with benzenesulfonic acid. The drug is provided in 2.5 ml, 5 ml and 10 ml ampules having a strength of 2 mg/ml cisatracurium besylate. In addition, a 30 ml vial containing 5 mg/ml cisatracurium besylate is also available.
Cisatracurium besylate slowly loses potency with time at a rate of approximately 5% per year under refrigeration (5° C.). Nimbex should be refrigerated at 2° to 8° C. (36° to 46° F.) in the carton to preserve potency. The rate of loss in potency increases to approximately 5% per month at 25° C. (77° F.).
Atracurium besylate is disclosed in U.S. Pat. No. 4,179,507 (hereinafter U.S. '507), which describes a series of bis veratryl isoquinolinium quaternary ammonium salts, including atracurium besylate. The synthesis of atracurium besylate, as taught in U.S. '507, involves the coupling of (±)-tetrahydropapaverine base, compound (II), with 1,5-pentamethylene diacrylate, compound Treatment of the resulting tertiary amine base with oxalic acid results in the isolation of N,N′-4,10-dioxa-3,11-dioxotridecylene-1,13-bis-tetrahydropapaverine dioxalate, compound (IV). The dioxalate salt (compound (IV) is converted to the free base, compound (V), with sodium bicarbonate solution and extracted into toluene. After evaporation of the toluene, the residue is dissolved in acetonitrile and treated with methyl benzenesulfonate. The addition of diethyl ether results in the precipitation of atracurium besylate, compound (VI), which is subsequently filtered and dried. Scheme 1 below illustrates the chemical pathway described above.

U.S. '507 teaches that the stereoisomerism of atracurium besylate (VI) may be partly controlled by the use of compound (II) of a defined stereochemical configuration and thus provide the tertiary amine base (V) of a RR-, SS-, or RS-(meso) configuration. The quaternisation process introduces another two centers of asymmetry with the resulting formation of a mixture of stereoisomers. However, the '507 patent makes no attempt at separating the mixture of stereoisomers.
Cisatracurium besylate is disclosed in U.S. Pat. Nos. 5,453,510 (hereinafter U.S. '510), and 5,556,978 (hereinafter U.S. '978), which describe forming (R)-tetrahydropapaverine from compound (II), and the conversion of compound (II) into a mixture of R and S diastereoisomer salts with the chiral amino acid, N-acetyl-L-leucinate, comprising 83% of the R and 17% of the S diastereoisomer. Crystallization of the mixture from acetone affords 97% (R)-tetrahydropapaverine-N-acetyl-L-leucinate and 3% (S)-tetrahydropapaverine-N-acetyl-L-leucinate, which is treated with aqueous ammonia and toluene to afford (R)-tetrahydropapaverine base, which is isolated from the toluene layer. The (R)-tetrahydropapaverine is subsequently reacted with 1,5-pentamethylene diacrylate followed by oxalic acid to afford the dioxalate salt of the bis tertiary amine base, (1R,1′R)-2,2′-(3,11-dioxo-4,10-dioxamidecamethylene)-bis-(1,2,3,4-tetrahydro-6,7-dimethoxyveratryl-isoquinoline, Conversion of the dioxalate salt to the bis tertiary amine base with sodium carbonate followed by treatment with methyl benzenesulfonate affords an aqueous solution of (1R,1′R)-atracurium besylate which is subjected to lyophilisation. The resulting pale yellow solid consists of a mixture of three isomers, namely, 1R-cis,1′R-cis; 1R-cis,1′R-trans; 1R-trans,1′R-trans (hereinafter referred to as the (1R,1′R)-atracurium besylate isomer mixture) in a ratio of about 58:34:6 respectively. The (1R,1′R)-atracurium besylate isomer mixture is subjected to preparative HPLC column chromatography on silica using a mixture of dichloromethane, methanol and benzenesulfonic acid in the ratio of 4000:500:0.25 as the eluent. The fractions containing the required isomer are collected and washed with water. The dichloromethane solution is evaporated to dryness, the residue dissolved in water and the pH of the solution adjusted to 3.5-4.0 with an aqueous solution of benzenesulfonic acid. The aqueous solution is lyophilized to afford cisatracurium besylate possessing an isomeric purity of about 99%. Both U.S. '510 and U.S. '978 describe performing the high-performance liquid chromatography separation in the presence of a strong acid which include benzenesulfonic acid, methanesulfonic acid, p-toluenesulfonic acid or phosphoric acid. In addition, an alcohol such as methanol, ethanol or n-propanol is required for the isomer separation.
However, HPLC methods for separating the isomers of (1R,1′R)-atracurium besylate that apply strong acids may be unsatisfactory for large scale production because stainless steel (commonly used in HPLC instruments) is not compatible with strong acids (such as benzenesulfonic acid) due to an excessive corrosion of stainless steel components resulting in the possible contamination of the product, which is also undesirable, especially on large scale. Therefore, there is a need for improved methods, that avoid using strong acids, for separating the (1R,1′R)-atracurium besylate isomer mixture, particularly on a large scale. The present invention provides such methods.